1. Field
The disclosed embodiments relate generally to the powering up and/or the powering down of a power domain within an integrated circuit.
2. Background
FIG. 1 (Prior Art) is a simplified top down diagram of an example of an integrated circuit die 1. Integrated circuit die 1 includes a peripheral ring of bonding terminals 2, and four domains of internal circuitry 3-6. These domains are called power domains because the circuitry within each domain can be powered or unpowered separately from the other domains.
FIG. 2 (Prior Art) is a simplified schematic diagram of the four power domains 3-6 of FIG. 1. The domains are powered via a supply voltage terminal 7 (denoted PWR), and a ground terminal 8 (denoted GND). The supply voltage is supplied to the various domains 3-6 by a supply bus 9. The supply bus is sometimes called a power bus. Supply bus 9 typically is in the form of a grid of conductors. The linear shape illustrated in FIG. 2 is presented just as an example to simplify the explanation of an operation of the power domains.
Although a supply bus such as supply bus 9 is typically fashioned as a wide strip of metal so that it will have good electrical conductor properties, the supply voltage bus nonetheless does have a small series resistance, a capacitance and a small inductance. The resistance and inductance are represented in the diagram with resistor and inductor symbols.
The various domains of circuitry 3-6 are grounded by a ground bus 10. Although the ground bus is also typically made of a wide strip of metal, it too has a small series resistance, a capacitance and a small inductance. The resistance and inductance are represented in the diagram with resistor and inductor symbols.
To reduce power consumption of integrated circuit 1, it is desired to be able to cut power to a domain if the circuitry within the domain is not in use. Switches 11-14 are provided so that the current path from power bus 9 to ground bus 10 through a selected domain can be cut if desired. If, for example, switch 13 is open, then there is no current flow from power bus 9 to ground bus 10 through domain 5. It is also desired to be able to supply power to a domain of circuitry that is to be used. If, for example, switch 13 is closed, then a supply current can flow from power bus 9 to ground bus 10 through domain 5.
FIG. 3 (Prior Art) illustrates a problem that can occur when a dormant power domain is then powered up so that circuitry within the domain can be used. Initially, circuitry within domains 3, 4 and 6 are in use. Switches 11, 12 and 14 are therefore closed. Switch 13, however, is open because the circuitry of domain 5 is not in use. Each domain has local power and ground conductors that extend to the various circuit elements within the domain. The local power and ground conductors and the circuitry within the domain constitute a capacitance between the power and ground buses 9 and 10. The capacitor symbols 15-18 in the domains represent these capacitances. Because switch 13 is open, capacitor 17 is discharged.
Next, the circuitry within domain 5 is to be used. The circuitry within domain 5 therefore needs to be powered. Switch 13 is closed so that the local power and ground bus lines within the domain can be charged and so that power and ground potential can be supplied to the circuitry within the domain. Initially, however, there is no charge on capacitor 17. A large inrush of current 19 therefore flows into the domain to charge the capacitance 17. This current 19 flows through supply bus 9, into domain 5, and out of domain 5 through ground bus 10. Due to the series resistance and inductance of the supply bus, the large current surge on supply bus 9 causes the voltage on node N2 to spike down. Similarly, the large current surge on ground bus 10 causes the voltage on node N6 to spike up. (Other nodes N1, N3, N4, N5, N7, N8 are also illustrated in FIG. 3.) These spikes reduce the magnitude of the voltage between nodes N2 and N6. Rather than the proper supply voltage being present between nodes N2 and N6, the supply voltage seen by the circuitry within domain 4 drops quickly. This may disrupt operation of circuitry in the neighboring power domain 4. A solution is desired.